1. Field of the Invention
This invention relates to apparatus for measuring properties or electrical characteristics in an electrical circuit and more particularly to a digital circuit for measuring relative frequency in separate pulse trains.
2. Description of the Prior Art
Various electronic instruments may produce digital pulse trains, or groups of pulses, in which the spacing between pulses or its reciprocal, the instantaneous frequency, is variable. The pulse trains are distinguished from one another by the fact that during the time between pulse trains only a very few pulses will be present. Pulse trains of this type may be produced by such devices as voltage-to-frequency or current-to-frequency converters handling input signals of a pulsating nature.
One common data-processing task to be performed on such pulse trains is the determination of the interval during which the instantaneous frequency exceeds a given threshold. This data-processing problem becomes more complex when this threshold must be adaptive. For example, the threshold frequency could be a pre-set fraction of the maximum frequency generated during an earlier pulse train. In the case where the input pulses arise from a current-to-frequency converter handling pulsating input currents, this data-processing task is equivalent to determining the time interval during which the input current exceeded a preset fraction of the peak input current in a previous current pulsation.
In the prior art, frequency-to-voltage converters and counter-computers were commonly used for similar data-processing tasks. A frequency-to-voltage converter (or frequency-to-current converter) would generate a continuously-varying output signal, which ideally would be proportional to the instantaneous input frequency. If the peak value of this analog output signal were to be sampled and stored for a given input pulse train, then the resulting reference voltage (or current) could then be compared to later signals from the converter in order to determine if they exceeded a pre-set fraction of the peak value of the first signal. Because a filter was necessary to smooth the output signal from the frequency-to-voltage converter and this filter must inevitably introduce timing errors, this approach functions accurately over only a limited range of input frequencies or requires an adaptive analog filter. Furthermore, this approach requires the careful adjustment which is typical of analog circuits and also is subject to their problems involving drifts and component uncertainties.
Thus, a digital approach would be preferable in order to eliminate these difficulties associated with analog circuits. For example, counter-computer circuits could be used to count the input pulses for known periods, thus determining the input frequency averaged over this period. Digital computation using these results could then determine such quantities as the maximum (average) frequency and the time during which the average frequency exceeded a given threshold.
Not only are such circuits relatively complex, but also they have limitations imposed by the rate at which samples can be made and the computations performed. These limitations become particularly severe if the time between pulses changes significantly on a pulse-to-pulse basis, so that the frequency averaged over several pulses becomes a poor representation of the instantaneous frequency. In that case, the counter-computer approach generates timing errors of the same nature as those resulting from the filtered output of a frequency-to-voltage converter.